In the battle against your immune system, some viruses may actually do you a favor by infecting your system before anything else. Researchers theorize that happens because your antiviral defenses are raised significantly when you are infected, potentially blocking other viruses from having a shot at going to war.

The report comes in the wake of a surprising lack of swine flu cases in Europe this fall — something researchers say might be blamed on the common cold (or rhinovirus) and its trigger of heightened defense in people. From New Scientist,

In France, flu cases rose in early September, then stayed at about 160 per 100,000 people until late October, when numbers started rising again. The delayed rise was puzzling, says Jean-Sebastien Casalegno of the French national flu lab at the University of Lyon. He reports that the percentage of throat swabs from French respiratory illnesses that tested positive for swine flu fell in September, while at the same time rhinovirus, which causes colds, rose. He told New Scientist that in late October, rhinovirus fell — at the same time flu rose. He suspects rhinovirus may have blocked the spread of swine flu via a process called viral interference.

So why has the U.S. fared so badly with swine flu even though cold season is in full swing? Researchers believe the concentration of H1N1 in the United States is so high that even with a cold, repeated exposure will eventually infect a person.

Moral of the story? Keep washing your hands. Snuggling up with someone infected with the common cold probably ain't worth it.